The invention relates to a device for the control of the wet application weight of coatings on material webs, in particular of conventional pigment dispersions on paper or carton webs.
In order to improve the printability of paper surfaces and to adapt the paper to particular printing processes, such paper surfaces are coated with pigment/bonding agent dispersions. Depending on the intended application the dry coating weights lie between 5 and 35 grams per square meter. The coating is conventionally carried out by means of aqueous dispersions with varying contents of drying substances so that the wet application weights are considerably higher. It has proved to be advisable to first apply an excess of coating material to the running web of material and to subsequently remove this excess from the surface by means of metering devices, whereby a smoothing of the coating surface likewise is an objective. Among the known metering devices are roll-type application arrangements using metering rolls, so-called reverse roll coaters, scraping roll metering devices, also known as rakels, and smoothing-and-scraping devices which are briefly known as blades.
In the metering process the coated web of material is carried by a supporting device, for example a roll, and a pressure is exerted on the web by the metering assembly, for example the blade or rakel which pressure opposes the coating mass.
In U.S. Pat. No. 2,051,403 there is described a coating arrangement the essential feature of which resides in the design of the wiping device. This especially formed, curved wiping device exhibits in the course of its curvature two different radii. The end of the entrance flank is not a sharp edge but it is rounded since sharp edges are said to lead to a faulty surface and to unsatisfactory results. The adjoining entrance flank is convexly curved and has a larger radius than the radius of curvature of the rounded edge.
In U.S. Pat. No. 2,534,320 a wiping device with a blade thickness of between 3 and 12 mm has been described the entrance flank of which ends in a sharp edge, with the radius of curvature of the entrance flank approximately corresponding to the radius of curvature, of 150 mm, of the roll supporting the web, this roll having an elastic surface. The curvature is convex in this design. The scraper is mounted in a rotatable support and it can be urged against the web surface with a varying pressure so that the surface of the supporting roll is deformed.
In U.S. Pat. No. 1,925,092 there is described a wiping device which is directed against a freely carried web, the web being carried, for a short length, around the curved entrance flank of the wiping device up to the sharp edged end of the latter. The wiping device simultaneously serves as a web guiding and deflecting element. The gap thus formed is bounded by the convexly curved surface of the wiping device and the concavely curved coated surface of the web. Since the outgoing web leaves the sharp edge of the wiping device tangentially, adjustment of a well-defined entrance gap is not possible. The geometry of the entrance gap formed in this manner depends not only on the web velocity but also on the quantity and the rheological properties of the coating mass as well as on the stretching properties of the paper web. The gap geometry, therefore, is not only defined by the device but it is dependent on the process conditions which exist at any given time and is thus more or less undefined and not controllable.
In principle this is true also for the known metering device using a doctor blade and a material web which is supported by a roll.
Depending on the magnitude of the pressure and the position of the metering device relatively to the surface of the coated material web, a layer of greater or lesser thickness remains on the material web. Doctor blades and also rakel rods are, taken by themselves, not sufficiently stable tools for the setting of a gap geometry and in addition they require mountings and guides which lend to them the required linearity, bending strength and resistance against the hydrodynamic counter-pressure of the coated mass. Particularly high requirements apply to the design of such mountings. With the web widths of several meters and with the web velocities of up to 1200 mm per minute which are common today the requirements can be met by a corresponding constructive design merely to a certain degree. As a result, the known metering devices are unsatisfactory particularly when, for the achievement of high application weights, the known doctor blades are laid against the web with a small setting angle and low application pressure. Under these conditions there is formed a wiping area or zone over which the wiping device engages the web surface. This leads, very soon, to a condition in which blade forces, partially can no longer withstand the higher coating pressure and in which a differing application weight is produced cross-wise of the web. Even small deviations form linearity, due to manufacturing tolerances or other causes lead to considerable variations in the application quantities cross-wise of the web following metering. Attempts have been made to improve the linearity by supporting, for example the doctor blade, by means of pressure hoses. These results are unsatisfactory especially in the case of larger web widths.
The coating systems possible and conventional today may exhibit operating widths of up to 6000 mm. The operating velocities and the application weights greatly depend on the paper types and the desired properties. For LWC (light weight coated) paper they range up to 1300 meters per minute with application weights of up to 10 grams per square meter and side; for cartons, for example, to 250 meters per minute with application weights of approximately 30 grams per square meter for each side, and for so-called art printing paper up to 600 meters per minute with application weights of approximately 25 grams per square meter and side.
In the course of systematic investigations it has been found that the gap geometry is of considerable significance and that it is desirable, therefore, to design this gap geometry in a well-defined way, and to control in this fashion the hydrodynamic pressure in the entire entrance gap.